Siemens SIMATIC PCS 7 Operating Manual page 175

CPU utilization
Before making any changes, check the CPU utilization with regard to memory and cycle
time. You can query how much memory has been allocated using the "Module Information..."
function in the CPU.
● To do this, open "Module Information..." in the CPU and check how much memory is
allocated at present on the "Memory" tab.
● To determine the cycle utilization, blocks TIME_BEG and TIME_ENG must be installed in
the OBs in use and interconnected with one another. In the relevant CFC, check the
runtimes of the OBs at output TM_DIFF_TI of the respective TIME_END block. Please
note that the runtimes of higher-priority OBs are also measured when this method of
measuring the runtimes of lower-priority OBs is used, and will need to be subtracted
accordingly.
With PCS V7.0 SP1 and higher, and an S7-400H CPU with FW V4.5 and higher, the
gross and net runtimes of the cyclic interrupt OBs can also be read at block CPU_RT.
Note
You can find more information in section Cyclic interrupts (Page 23).
Checking the H parameters
In fault-tolerant systems, you must check the H parameters of the CPU.
● To do this, open the Object Properties of the CPU and switch to the "H Parameters" tab.
● If a "Cyclic interrupt OB with special handling" has been specified here, and the option
"Use only calculated values" has been activated, use the "Calculate..." button to open the
"Update Reserve: Calculation of Monitoring Times" window.
● Here, check the values in the entry fields "Runtime of the watchdog interrupt concerned
[ms]" and "Work memory used for all data blocks in the user program [Kbyte]".
● Compare the runtime entered in the "Runtime of the watchdog interrupt concerned [ms]"
field, for the cyclic interrupt OB with special handling, with the measured runtime
(TIME_BEG, TIME_END of OB) or the runtime read at the CPU_RT block. The value
entered should be approximately 10% greater than the measured one.
● Compare the value for "Work memory used for all data blocks in the user program
[Kbyte]" with the object properties of the CPU block folder. There, in the "Blocks" register
under "Size in work memory: Data", you will find the amount of memory that is occupied
by data blocks (in bytes). This value (expressed in Kbytes), plus some reserve, must be
entered in the "Work memory used for all data blocks in the user program [Kbyte]" field.
● If the values in the entry fields do not match the actual values, you must enter the current
values and determine the H parameters using the "Calculate..." button.
Note
You can find more information in section Calculating monitoring times (Page 36).
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB
Changes, tracking changes, and acceptance
9.2 Preparing for changes
175